How come ears work but cubes don't? (See http://www.renderosity.com/messages.ez?Form.ShowMessage=2105489)

It seems to make sense that the side of a cube towards the light casts a shadow on the side away from the light, thereby blocking any translucent effect. But wouldn't that be true for a 3D ear as well?

The picture is a ball and cube primitive, each with a diffuse node linked to their translucent nodes. The ball at full scale shows no translucence, and the cube shows some near the top edge (left).

But reduce the Z Scale to 3 percent on each (right), and the translucence shines through! The distance between the surfaces seems to matter, and ears are thin where the light shines through.